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February 25, 2011

Global Fire Ant Invasion Traced To Southern US

by Sam Savage

New research from the University of Florida has found that red fire ant invasions around the world in recent years can now be traced to the southern United States, where the insect gained a troublesome foothold in the 1930s.

Native to South America, the ant has been believed to be contained there and in the southeastern US, before turning up in far off place over the past two decades. Genetic tests on ants in California, China, Taiwan, Australia and New Zealand has traced them back to southeastern US, according to an international team of researchers reporting in the journal Science.

The aggressive stinging ants arrived in the US from South America in the 1930s, and then gradually spread out across the South. Now, the US has become the catalyst for the ants to invade other countries.

The genetic profile of the ants abroad is much closer to fire ants in the US, explained Marina Ascunce of the Florida Museum of Natural History at the University of Florida.

"I thought that at least one of the populations in the newly invaded areas would have come from South America, but all of the genetic data suggest the most likely source in virtually every case was the southern U.S.," she said in a phone interview with The Associated Press.

Scientists hope the discovery will help prevent further spread of the invasive species, Solenopsis invicta. Americans spend more than $6 billion a year to control the ants and offset the damage they cause, including medical expenses and more than $750 million in agricultural losses.

"By knowing where they are coming from, biological controls can be more focused," said Ascunce. "We can also improve screening or monitoring in source areas or on key transportation routes."

Red imported fire ants are very aggressive. They have a painful sting, often discovered by humans only after stepping on a mound. "People who are allergic can die" from the ant stings, said Ascunce.

The researchers used several different types of molecular genetic markers to trace the origins of the ants in the nine locations where invasions occurred. All but one were traced back to the southern United States. The exception was an instance where the ants had moved from southeastern US to California, then on to Taiwan.

The study results show the problematic side of a strong global trade and travel network.

DeWayne Shoemaker, a U.S. Department of Agriculture scientist affiliated with UF's Institute of Food and Agricultural Sciences who was a lead investigator on the grant that funded the study, said finding the precise origins for the ants is a big victory because it helps scientists know where to look to find the most effective biological control agents, such as phorid flies.

Scientists have been releasing phorid flies around ant colonies since the late 1990s, reducing the use of pesticides. The flies hover over mounds before injecting an egg into an ant, which then hatches, kills the ant, and then the cycle repeats itself.

Shoemaker, a key member of the research team that sequenced the complete genome of the red imported fire ant earlier this year, said the team collected ants from 2,144 colonies at 75 geographic sites. They then used multiple genetic tests to determine the ants' origin.

"I really think our power to distinguish "¦ hinged on us having such a large data set," said Shoemaker. "I don't think we'd have had the statistical power to come up with these kinds of conclusions otherwise. All of these conclusions are highly supported by data."

"Although those of us who research fire ants are not surprised at the finding, this is the first hard evidence of the sources of worldwide fire ant spread," Larry Gilbert, director of the Brackenridge Field Laboratory at the University of Texas at Austin, told AP.

"Various exotic invasive pests have been and are being transported to the U.S. by commerce, so it's ironic that one of our pests has used the southern U.S. as staging area, and then as a port of exit for worldwide destinations. This is a sad and disturbing result, but one that needs to be addressed, not only in the U.S. but in more recently infected countries," said Gilbert, who was not part of the research team.

He said the study results highlights the need to find the most common ways ants travel across great distances to develop better ways to intercept the pests.

Ascunce noted that monitoring in South America has blocked shipments from that region to Asia when fire ants were discovered.

The study was supported by the U.S. Department of Agriculture, Taiwan Council of Agriculture and National Science Council of Taiwan.